Magnetization dynamics is a key aspect of how magnetic technology based devices function. How the magnetization precesses around the effective magnetic field and how this precession is damped influences how fast and how energetically efficient a magnetic memory bit is switched in magnetoresistive random access memory (MRAM). The important magnetic parameters that describe these processes are the damping coefficient, intrinsic magnetic anisotropy, saturation magnetization.
A lot of research goes into finding the magnetic materials with the right parameters, but measuring the parameters typically involves either patterning the film into a device with submicron size that can be electrically characterized at microwave frequencies or breaking a piece of the film and inserting it into a microwave waveguide to study the ferromagnetic resonance. Both of these methods are costly in time and resources. A method that is preferably nondestructive is desired for obtaining timely feedback in the material selection as part of the design process and for monitoring of fabrication process at full film wafer level.
Worledge, et al. have described a method for measuring magnetoresistance (MR) and resistance area product (RA) of unpatterned magnetic tunnel junction film stacks. The RA is measured by making a series of four-point probe resistance measurements on the surface of an unpatterned wafer at various probe tip spacings. The probe tips are spaced apart on the order of microns for typical applications. The MR is obtained by repeating the measurement while applying different magnetic fields. (Worledge, et al.; Magnetoresistance measurement of unpatterned magnetic tunnel junction wafers by current-in-plane tunneling, Applied Physics Letters, Vol. 83, No. 1, 7 Jul. 2003, pp. 84-86; and U.S. Pat. No. 6927569.)
Commercially available automated metrology tools (e.g. from CAPRES), which are designed for measuring selected magnetic parameters of unpatterned MTJ film stacks, use multi-point probes with probe tip spacings in the micron range. Four- and twelve-point probes are available for these automated metrology tools. As an example, a CAPRES twelve-point probe is used with a 12-by-4 multiplexor (MUX) to select a total of 495 different pin-configurations each with different probe spacings (pitch). This approach allows the selected tests to be performed with different probe spacings without having to have the tips be movable with respect to each other. Existing automated metrology tools also provide means for applying a selected magnetic field to the test sample.
Y. Suzuki, et al. have discussed a “spin-torque diode effect” in patterned magnetic tunnel junction cells. A spin-polarized radio frequency (RF) current injected into a magnetic cell through the top and bottom electrodes exerts a torque to the local spin momenta and may excite ferromagnetic resonance (FMR) modes in the magnetic cell. FMR mode excitation in a magnetic tunnel junction is accompanied by the oscillation of its resistance and results in a rectification effect. The “spin-torque diode effect” provides a quantitative measure of the spin-torque, and it can be used to extract magnetic parameters of the cell. (Yoshishige Suzuki and Hitoshi Kubota; Spin-Torque Diode Effect and Its Application; Journal of the Physical Society of Japan, Vol. 77, No. 3, March, 2008, 031002.)
In US patent application 20100033881 Carey, et al. describe a magnetic field sensing system (usable in a head in disk drive) that uses a “spin-torque diode effect” that has been identified in magnetic tunnel junctions (MTJ). The effect is said to generate a measurable direct-current (DC) voltage across a MTJ device when an RF alternating current (AC) is applied with a frequency that is approximately the FMR frequency of the free layer. The explanation states that when the free layer and reference (pinned) layer magnetizations are in the film plane and are oriented at an angle, the AC passing through the layers of the MTJ device exerts an alternating torque on the free layer magnetization, rotating it towards and then away from the pinned layer magnetization during the complete AC cycle. However, the resistance is higher for electron current flows in one direction than the other. The resistivity of the whole layer stack changes when the magnetization of the free layer changes direction relative to that of the pinned layer, exhibiting a low (high) resistance state when the magnetization orientation of the two ferromagnetic layers in substantially parallel (anti-parallel) direction. The change in resistance follows the AC signal frequency (with a phase lag) when the frequency is sufficiently close to the free layer resonance frequency. The result is a DC output voltage that is proportional to the AC current. An external magnetic field allows the frequency window around the free layer resonance frequency to be tuned. In the embodiment described by Carey, et al. the AC current is applied in a conventional manner through the top and bottom electrodes in a fully patterned (completed) MTJ in a read head.